1. Field of the Invention
The subject invention generally pertains to tube and shell heat exchangers and more specifically to an evaporator that provides a chiller water temperature marginally above freezing.
2. Description of Related Art
Many chiller systems include a closed loop refrigerant circuit comprising a compressor, a condenser, a flow restriction, and an evaporator. Expanded, cold refrigerant in the evaporator cools a secondary closed loop chilled fluid circuit. The chilled fluid, such as water or a water-based solution, is distributed to and circulated through various smaller heat exchangers. The smaller heat exchangers cool various comfort zones, such as rooms or other areas within a building.
In many cases, one or more chillers are dedicated to a single building. However, in some cases one large central cooling system, comprising one or more chillers, serve several distinct buildings. The chilled water is typically piped a great distance to reach the various buildings. Such a chiller system is often referred to as a "district cooling system."
As chilled water is conveyed through a relatively long network of pipes, the water takes on heat before reaching its various designated heat exchangers. To ensure that the chilled water is sufficiently cold upon reaching the heat exchangers, it is usually desirable to have the evaporator reduce the temperature of the water as much as possible. However, if the water gets too cold, it may freeze inside the evaporator. Freezing, of course, can destroy the evaporator and/or its associated piping.
To avoid freeze up, the chilled water solution may be a glycol and water solution or some other solution having a lower freezing point than pure water. However, with district cooling systems, an appreciable amount of glycol or other solution that may lower the freezing point can be rather costly due to the large volume contained within the chilled water piping that interconnects the evaporator and the remote heat exchangers. Consequently, current district cooling systems use water solutions that consist of primarily water with perhaps small amounts of water treatment chemicals. Since such solutions have a freezing point near 32 degrees Fahrenheit, evaporators are typically operated at a temperature safely above that.
To this end, many chillers control the leaving chiller water temperature (LCWT) in response to a temperature sensor installed immediately downstream of the evaporator or situated within an outlet water box of the evaporator (see U.S. Pat. Nos. 5,083,438 and 5,355,691). The outlet water box serves as somewhat of a manifold or collection point into which the numerous heat exchange tubes within the evaporator shell discharge. The temperature sensor, whether in the water box or immediately downstream of the evaporator, usually provides a generally good indication of the LCWT.
However, the sensed temperature is only an average of the actual water temperature discharging from each individual tube of the evaporator. In a tube and shell heat exchanger the discharge temperature at each tube often varies from one tube to the next, depending on its location within the shell and the conditions under which the system is operating. Thus, to avoid freeze up at any individual tube, chillers are usually controlled to provide an average LCWT that is well above freezing, typically 37 degrees Fahrenheit or higher.
Unfortunately, when leaving the evaporator at 37 degrees, the chiller water temperature may rise to an unacceptable high temperature by the time it reaches the remote heat exchangers of a district cooling system.
In some chiller systems, such as the one disclosed in U.S. Pat. No. 5,782,131, a temperature sensor senses the temperature of the refrigerant inside an evaporator, as opposed to directly sensing the temperature of the chilled water. However, with such a system it may be difficult to determine what minimum allowable refrigerant temperature still avoids freezing the water. For example, in some cases, a refrigerant temperature of 30 degrees might only be able to chill the water to 38 degrees Fahrenheit.